crawford



Feb. 14, 1956 J CRAWFORD 2,735,020

CURRENT TRAP FOR PREVENTING COUPLING BETWEEN TWO GENERATORS Filed June9, 1955 INVENTOR. JACK A. CRAWFORD ZQWQQM 5.5,. 60 M44,

ATTORNEYS United States Patent CURRENT TRAP FOR PREVENTING COUPLINGBETWEEN 'TWO GENERATORS Jack A. Crawford, Inyokem, United States ofAmerica Secretary of the Navy Calif., assignor to the as represented bythe The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to electrical circuits, and in particular toimprovements in a current trap for preventing coupling between twogenerators supplying a common load such as is disclosed in the U. S.patent application of .Irvin H. Swift, Serial No. 360,617, filed June 9,1953.

It is an object of this invention to provide a current trap which willprevent interaction between two series connected generators having acommon load impedance, and which requires a minimum number ofcomponents.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same become better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a schematic illustration of one form of the invention, and

Fig. 2 is a schematic illustration of a second form of the invention.

In Fig. 1, two sources of A. C. potential, or generators, 10, 11 havinginternal impedances, 12, 13 respectively, are connected in series withload impedance 17 having a value Z1. Autotransformer 19 is connected inparallel with load impedance 17, and impedance 21 is connected betweenthe center tap 22 of autotransformer 19 and ground 23. The value ofimpedance 21 substantially equals Z1/4. Terminals 25 and 26 ofgenerators and 11 are likewise connected to ground 23.

The generated voltage of source 10 is E1, and the generated voltage ofsource 11 is E2. The frequencies and wave shapes of E1 and E2,respectively, are not restricted. The instantaneous value of the output,or terminal, voltage of source 10, 21, is applied to terminal 27, whilethe instantaneous value of the output, or terminal, voltage of source11, e2, is applied to terminal 28. The instantaneous value of thepotential of center tap 22 with respect to ground 23 is assumed to beex. Then by transformer theory, it is known that the potentialdifference between terminal 27 and center tap 22 is equal to andopposite the potential ditference between terminal 28 and center tap 22so that The value of the current is flowing between center tap 22 andground 23 through impedance 21 is e 1+ 2) (3) Vi/4 21 where Z1 is thevalue of impedance 17.

Since the currents in each half of the autotransformer 19 are equal, asis known from transformer theory, it

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being assumed that the magnetization current is negligible, it followsthat the current flowing through each half of autotransformer 19 isequal to The current iii flowing through generator 10 equals the currenti4 flowing through load impedance 17 plus the current ia/2 flowing .fromterminal 27 to center tap .22. Therefore,

( ii=i4+is/2 and where Z1 is the value of load impedance 17.Substituting Equations 4 and 6 in Equation 5 yields 1 By circuitsymmetry it is also known that iz, the current flowing through generator11 is equal to From the foregoing it can be concludedzthat the currentthrough generator 10, for example, .is determined 'solely by the outputvoltage of generator '10. From this it follows that the currents i13'nd'l 2 are unaffected by voltages ea and er respectively, so that theterminal voltage of each generator Q1 and eg is independent .of thevoltage of the .other generator.

In Fig. 2 there is illustrated a second embodiment of the invention inwhich the primary coil of the transformer performs the function ofautotransformer 10 of Fig. 1.

Two sources of A. C. potential, or generators, 110, 111 having internalimpedances 112, 113, respectively, or connected to have the primary coil114 of transformer 115 as a load common to both. Load impedance 117having a value of Z10 is connected across the secondary coil 116 oftransformer 115. Coil 114 has N1 turns, and coil 116 has N2 turns. Coil114 has a center tap 122. Impedance 121 which has a value substantiallyequal to Z1o/4(N1/N2) is connected between center tap 122 and ground123. Terminals 125 and 126 of generator 110 and 111 are likewiseconnected to ground 123.

The generator voltage of source 110 is E10, and the generated voltage ofsource 111 is E20. The frequencies and wave shapes of voltages Em andE20 are not restricted. The instantaneous value of the terminal, oroutput voltage of source 110, cm, is applied to terminal 127, while theinstantaneous value of the terminal, or output, voltage of source 111,can, is applied to terminal 128.

The instantaneous value of the potential of center tap 122 with respectto ground 123 can be assumed to be e For the reasons set forth inexplaining Fig. 1, it can be shown that Knowing the potential ey interms of em and 220, the value of the current 1'30 flowing betweencenter tap 122 and ground 123 through impedance 121 is then 3 whereZ1o/4(N1/N2) is the value of impedance 121. Since the currents in eachhalf of coil 114 are equal, as is known from transformer theory, itbeing assumed that the magnetization current is negligible, it followsthat the current flowing through each half of coil 114 is The currentin) flowing through generator 110 equals the current in: flowing throughcoil 114 plus the current fan/2 flowing from terminal 127 to center tap122. Therefore,

where Z(N1/N2) is the reflected value of the impedance 117. SubstitutingEquations 13 and in Equation 14 yields z1t N1/N2 By circuit symmetry itis also known that izo, the current flowing through generator 111 isequal to The same conclusions as to the operation of the circuit of Fig.2 are reached as were reached concerning the 1. An electrical circuitcomprising a transformer having primary and secondary coils having N1and N2 turns, respectively, said primary coil having two terminals and acenter tap, a load impedance connected across the secondary coil of thetransformer, first and second sources of A. C. potential, each sourcehaving two terminals, one terminal of the first source of A. C. beingconnected to one terminal of the primary coil of the transformer, oneterminal of the second source of A. C. being connected to the secondterminal of the primary coil of the trans former, the other terminals ofthe first and second sources being common, a second impedance connectedbetween the center tap and the common terminals, said second impedancehaving a value substantially equal to onefourth the value of the loadimpedance multiplied by the square of the ratio of the number of turnsof the primary coil to the number of turns of the secondary coil of thetransformer.

2. An electrical circuit comprising a load impedance having twoterminals, a first source of A. C. potential and a second source of A.C. potential, said sources each having two terminals, one terminal ofthe first source being connected to one of the terminals of the loadimpedance, one terminal of the second source being connected to theother terminal of the load impedance, the other terminals of the sourcesbeing joined, an autotransformer having a center tap connected inparallel with the load impedance, and a second impedance connectedbetween the center tap of the autotransformer and the joined terminalsof the first and second sources, said second impedance having a valuesubstantially equal to one-fourth the value of the load impedance.

References Cited in the file of this patent UNITED STATES PATENTS937,469 Rose Oct. 19, 1909 1,373,923 Unger Apr. 5, 1921 2,250,308Lindenblad July 22, 1941 OTHER REFERENCES Alternating Current Phenomena,by Steinmetz, McGraw-Hill Book Co., 1916; pages 292-3.

